1. Field of the Invention
The invention relates to a receiver provided with a frequency control loop for automatic fine tuning, consecutively incorporating a controllable oscillator which is tunable in frequency by means of a variable capacitance diode, a mixer stage to which the signal from the controllable oscillator is applied for converting an RF receiver signal into an IF signal, a frequency voltage converter tuned to a fixed intermediate frequency for converting frequency deviations of the IF signal relative to the fixed intermediate frequency into amplitude variations of the frequency voltage converter output signal relative to a reference value, a control signal generating circuit for deriving a control signal for the variable capacitance diode of the controllable oscillator from the output signal of the frequency voltage converter, said controllable oscillator at least partly compensating its asymmetrical capacitance-voltage characteristic.
2. Description of the Prior Art
A frequency control loop for automatic fine tuning for use in a receiver of this type is known from the published German Patent Application No. 1,260.558.
The control signal generating circuit of the known frequency control loop comprises a transistor which amplifies on the one hand the output voltage of a frequency discriminator functioning as a frequency voltage converter in order to increase the control slope and on the other hand drives the variable capacitance diode in such a manner that its capacitance varies linearly with the said frequency deviations of the IF signal in the case of normally occurring field strengths.
This variation is achieved in the known frequency control loop by operating and choosing the operating point of the transistor such that positive and negative frequency deviations for normally occurring field strengths result in an amplitude limitation of the control signal supplied by the transistor at first and second fixed values, respectively, which are located asymmetrically around the said reference value corresponding to the intermediate frequency. Under certain circumstances the asymmetry thus obtained in the control signal compensates the asymmetry of the capacitance voltage characteristic of the variable capacitance diode of the controllable oscillator. As a result the capture and hold range of the frequency control loop, hereinafter breifly referred to as the control range, is to a certain extent located symmetrically around the correct tuning point, that is to say, the tuning position at which the carrier of the IF signal coincides with the said fixed intermediate frequency.
In the tuning ranges where amplitude limitation of the control signal occurs, the control signal does not vary with possibly occurring frequency deviations of the If signal and the automatic fine tuning is therefore not operative. Consequently a stable tuning position on the edges of the RF filter of the receiver preceding the frequency-voltage converter, also referred to as side tuning, becomes possible. This results in a considerably audible signal distortion.
In addition a linear amplification of the principally symmetrical discriminator voltage is effected for small field strengths in the transistor of the known frequency control loop so that also the control signal is symmetrical. The asymmetrical capacitance voltage characteristic of the variable capacitance diode is then not compensated so that also the control range of the known frequency control loop is asymmetrical.